Doug Fleenor Design DMXOPTO-5 - Manual (Page 10)

9
OPERATION
The DMX Opto Isolator has no operator controls; it is fully
automatic. It should be left plugged in at all times for
automatic operation. When a DMX signal is applied to the input,
it is isolated by the optocoupler and instantly (within
1/10,000,000 second) retransmitted by the output driver. When
the signal is removed from the input, the optocoupler is idle,
however the output driver continues to drive the DMX line in an
idle (high) state as long as the isolator is powered. The high
state is the idle state as defined by the DMX512 specification.
For power savings, the Converter may be unplugged, or plugged
into a switched outlet, when not in use. There is no special
sequence required in powering up the dimmers/console/isolator.
The isolator has one indicator LED. This indicator is
illuminated when a DMX signal is present on the input of the
isolator. It is not a "power" indicator. It is normal for the
indicator to change intensity slightly as dimmer levels are
changed. This effect may be so slight that it is not noticeable.
TECHNICAL DETAILS
For those technical types out there, here is a brief description
of how the DMX Opto Isolator works.
Differential Digital Signals
DMX512 uses a "differential" digital signal. A differential
signal is a pair of signals which are inverse from one another.
That is, when one is high, the other is low. Differential
signals are common in both analog processing (balanced microphone
cables) and digital processing (digital communications such as
EIA-485). Differential signals are used to reduce the effect of
electrical noise on long cable runs. When a differential signal
is processed by the receiving circuit (microphone pre-amp or RS-
485 receiver for example) the noise can be eliminated but the
signal remains.
Differential signals always travel in pairs, and the wire on
which they travel are usually twisted to insure any noise picked
up is of equal magnitude on both wires. The noise must be of
equal magnitude on each wire so that the receiving circuit can
effectively subtract the noise and leave the signal. The pairs
of signals are usually denoted by a plus (+) sign on the normal,
or "hot", or "true" line and by a minus (-) sign on the inverted,
or "return", or "complement" line.
In DMX512, a third line, common, is run in addition to the
differential signals. The DMX Opto Isolator ignores the common
signal all together, and uses only the relative state of the
differential signal. When the +data line is higher than the -
data line the LED indicator is illuminated and the output driver
drives the +data line high, the -data line low.